In one prior art screwdriver, a body of the screwdriver is installed with a cylinder, a piston rod, a pneumatic motor, etc. The piston rod is movably installed to the cylinder. The piston rod has a plug for opening or closing the motor air inlet valve. Thereby, high pressure air in the cylinder can push the pneumatic piston rod to linearly reciprocate and the high pressure air can drive the pneumatic motor to rotate and thus to drive the moving piston rod to rotate. Meanwhile, the plug on the piston rod can open or close the motor air inlet valve so as to control the motor to rotate or stop. Thereby, the piston rod can rotate and displace linearly as desired for locking a screw nail.
However, in above mentioned prior art, the plug for controlling the rotation of the motor is arranged on the piston rod which is rotatable and linearly movable. When the plug moves to motor air inlet valve for closing the valve opening, the plug of the piston rod still rotates, as a result, the air stop washer of the plug will wear easily so as to reduce the lifetime of the plug. Thereby, the prior art design has a short lifetime. These prior arts still exist in other prior art and can not be over-come effectively.